The invention relates to a pressure-limiting valve including a valve seat and a sealing element, both located in an inner space of the valve housing, and a spring for biasing the sealing element into an engagement with the valve seat, with the biasing force of the spring being adjustable to correspond to a desired opening pressure of the pressure-limiting valve.
The invention also relates to a method of setting the opening pressure of the pressure-limiting valve by adjusting the biasing force of the spring.
Pressure-limiting valves of this type are used in hydraulic delivery pumps, for example, in a hydraulic pump for delivering fluid to a power-assist steering gear of a motor vehicle. The purpose of providing pressure-limiting valves consists in by-passing excess hydraulic fluid or oil when a predetermined fluid pressure in the power-assist steering gear is exceeded, so that the power-assist would not exceed a predetermined threshold.
This conventional pressure-limiting valve includes a valve seat and a sealing element both located in an inner space of the valve housing, with the sealing element being spring-biased against the valve seat. The valve seat has a through-opening through which the hydraulic fluid flows for displacing the sealing element against the biasing force.
The sealing element usually includes a ball-shaped or conical sealing body, which engages the valve seat, closing the mouth of the through-opening. Conventionally, the valve seat is screwed into the valve housing displacing the sealing element and compressing the biasing spring. Thereby, the biasing force which is applied by the spring to the sealing element is adjusted.
The inner space of the valve housing has at least one bore, which communicates the inner space, in which the valve seat and the sealing element are located, and which has a valve outlet that communicates, for example, with a reservoir of the delivery pump.
In order for the pressure-limiting valve to function in a predetermined manner, i.e., to open when a predetermined pressure limit is exceeded, the pressure-limiting valve need be set to open at the predetermined pressure limit.
To this end, the valve seat is screwed into the valve housing to a predetermined position displacing the sealing element, together with the biasing spring, a distance corresponding to a predetermined preload of the spring.
The displacement threaded path of the valve seat is determined by using an appropriate number of adjusting washers, which are formed as U-washers, with which a distance between a stop provided in the valve housing and the locating face of the valve seat is adjusted.
By screwing-in the valve seat a pre-determined length, the spring is so preloaded that it applies a predetermined force to the sealing element for retaining the sealing element in engagement with the valve seat.
The sealing element is displaced away from the valve seat, against the biasing force of the spring, only after the oil pressure acting thereon reaches a predetermined value so that the valve becomes open. The oil, which then enters the valve inner space, can then flow to the reservoir.
The drawback of these conventional pressure-limiting valves consists in that the preloading of the spring is not very precise, because it is determined by the thickness of the adjusting washers and, thus, can be effected only in stages.
In addition, the use of adjusting washers complicates the adjusting process because, whenever one or several washers need be added or removed, the valve seat need be completely removed from the valve housing.
Accordingly, an object of the invention is a pressure-limiting valve which would have a simple structure and in which the adjustment of the biasing force of the spring can be effected with more precision than in conventional pressure-limiting valves.
Another object of the invention is a method of setting an opening pressure for the pressure-limiting valve according to the present invention.